Apparatus for measuring sulfur dioxide concentrations

ABSTRACT

Sulfur dioxide concentrations present in a gaseous mixture are rapidly and continuously monitored by measuring the current passing between an inert metallic sensing electrode and a counter electrode which electrodes are in contact with an aqueous electrolyte solution and at which sensing electrode sulfur dioxide is electrooxidized. The sensing electrode is composed of an inert metal whereas the counter electrode is composed of an electroactive material which is capable of being electrochemically reduced when electrically interconnected with the sensing electrode in the presence of the aqueous electrolyte solution.

United States Patent [72] inventors Ramesh Chand;

Manuel Shaw, both of Los Angeles, Calif. [21] Appl. No. 758,527 [22]Filed Sept. 9, 1968 [45] Patented Nov. 23, 1971 [7 3] AssigneeDynasciences Corporation Los Angeles, Calif.

[ 54] APPARATUS FOR MEASURING SULFUR DIOXIDE CONCENTRATIONS 8 Claims, 2Drawing Figs.

[52] US. Cl 204/195, 204/ 1 T [51] Int. Cl. GOln 27/46 [50]FieldotSearch 204/1.1, 195

[ 56] References Cited UNITED STATES PATENTS 310,302 1/1885 Moebius204/83 2,805,191 9/1957 l-lersch 204/l.1 2,844,532 7/1958 White et al.204/195 3,071,530 1/1963 Neville 204/195 3,088,905 5/1963 Glover.....204/195 3,213,004 10/1965 Schmidt 204/83 3,223,597 12/1965 Hersch 200/13,227,643 1/1966 Okun et al. 204/195 3,325,378 6/1967 Greene et al.204/l.1 2,9l3,386 11/1959 Clark 204/195 3,3 34,623 8/1967 Hillier et al.204/195 Primary Examiner-T. Tung Attorney- Donald E. Nist i a Z Z.

2a 19 22 25 kzokzs PATENTEnunv 23 IBM 3, 622 ,488

' II/I/I/I/ a JV I l b Z 1 Fl G 2 INVENTORS EL SHAW BY SH CHAN!)ATTORNEYS APPARATUS FOR MEASURING SULFUR DIOXIDE CONCENTRATIONSBACKGROUND OF THE INVENTION Sulfur dioxide is a serious atmosphericpollutant. According to the U. S. Public Health Service amounts of up toabout millions of tons per year are expelled into the atmosphere therebymaking it second only to carbon monoxide as a major source of pollutant.Sulfur dioxide is known to be extremely dangerous in view of itscorrosive and poisonous characteristics. This gas causes irritation andinflammation of the conjunctiva of the eyes and also afiects the upperrespiratory tract and the bronchi. The inhalation of sufficientquantities of sulfur dioxide may cause edema of the lungs or glottis andmay result in respiratory paralysis. In moist air or fogs the gascombines with water to form sulfurous acid which is slowly oxidized tosulfuric acid. Concentrations of less than about one part per million(p.p.m.) are believed to be injurious to plant life. Whileconcentrations of 400-500 p.p.m. may result in fatality, amounts betweenabout 50 and 100 p.p.m. are considered to be the maximum permissibleconcentration for exposures of to 60 minutes. The importance ofcontinuous monitoring of this pollutant is obvious.

Although a number of sulfur dioxide analyzers are presently availablethese instruments are less than satisfactory for a number of reasons.Such instruments are generally of high cost and bulkiness therebyinhibiting their widespread use and acceptance. In addition, many ofthese analyzers can only be operated by skilled technical personneloften requiring a number of steps in effecting the analysis as well ascritical calibration procedures and handling of chemical solutions.Further, instrumental response is often slow.

The most common method of analyzing for sulfur dioxide is bycolorimetric analysis which relies on absorption of the gas in achemical reagent thereby changing its color, the change being measuredphotometrically. Some disadvantages include considerable maintenance,poor response the bulky apparatus. Reagent solutions must be stored andconstantly pumped and samples require rigorous conditioning to removeinterfering species. Response time of such instruments is of the orderof 15 minutes for 90 percent response. The size and operation of theinstrument is such as to discourage its use outside the laboratory.

Electrolytic and thermoconductivity analyzers, although requiring lessmaintenance, are highly nonspecific, and are only suited to themeasurement of laboratory samples which have been conditioned to removeinterfering species.

Perhaps the most suitable analyzers presently available are thoseutilizing photometric means in which the infrared and ultravioletabsorbance properties of sulfur dioxide are monitored. With properfilter selection, such an instrument can be made specific for sulfurdioxide. However, the size of the analyzer makes it bulky to handlethereby discouraging its use as a portable-type instrument. Further,samples must be carefully handled and continuously conditioned in orderto eliminate foreign particles which settle out at the cell windows andthereby affect the sensitivity of the instrument. In addition, changesin light source intensity and detector tube sensitivity also affect themeasurements thereby necessitating frequent calibration checks.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a schematic crosssection of the transducer of the invention; and

FIG. 2 shows an enlarged detailed view of a broken away portion of thesensing electrode area of FIG. 1.

DESCRIPTION OF THE INVENTION It is to the elimination of the problemsgenerally associated with the above-noted methods of analyzing forsulfur dioxide that the present invention is directed. Specifically, theapparatus of the invention for measuring sulfur dioxide concentrutionspresent in a gaseous mixture comprises a transducer having a sensingelectrode and a counterelectrode which contact an aqueous electrolytesolution. The transducer is constructed in such a manner that thesensing electrode, at which sulfur dioxide is electrooxidized, islocated between the surface of the electrolyte which is in contact withthe sulfur dioxide containing atmosphere and the counterelectrode. Inthis manner, as the gaseous mixture is diffused into the electrolyte atthe gas-electrolyte interface, the sulfur dioxide molecules will contactthe sensing electrode and thereupon be electrooxidized to sulfate ions.At the same time the counterelectrode, which is in contact with theelectrolyte solution and interconnected with the sensing electrode so asto allow a current to flow therebetween, is electroreduced to consumethe sulfate ions produced by oxidation at the sensing electrode.

Initially, the operation of the transducer of the invention will be morereadily appreciated and understood by referring to the schematicrepresentation of the device as shown in the accompanying drawings.

FIG. 1 shows a schematic cross section of a preferred construction ofthe transducer of the invention whereas FIG. 2 illustrates, in detail anenlarged portion of the device at the sensing electrode. Each of thesedrawings may be referred to in the following description. Theelectrolyte solution 11 is contained in a suitable vessel 10 in which ispresent a counterelectrode l2 and a sensing electrode 13. The vessel 10may consist of any suitable material such as plastic, glass, etc. whichis preferably impact resistant and shatterproof. A means for passing acurrent between the sensing and counterelectrodes comprises a conductivewire 14. The wire 14 is attached to terminals 22 and 23 located on theexterior of the vessel 10 with the tenninals 22 and 23 beinginterconnected to the respective electrodes via conduits l9 and 20. Itwill be appreciated that other suitable means for electricallyconnecting the electrodes may be used which allows a current flow, whichcurrent is caused by electrons generated at the sensing electrode atwhich an electrooxidation reaction occurs. In a preferred embodiment, asshown in the drawings, the surface of the electrolyte is covered with asemipermeable membrane 17 through which a gas may diffuse but which willprevent significant losses of electrolyte by evaporation or spillage.However, in a simplified device, the membrance may be eliminated and theelectrolyte surface fully exposed to a gas. As sulfur dioxide containinggas contacts the electrolyte surface, sulfur dioxide molecules willdiffuse into the electrolyte with the concentration of sulfur dioxideinitially entering the electrolyte solution being proportional to itsconcentration within the atmosphere. Thereafter, as sulfur dioxidemolecules in solution in the electrolyte contact the surface of thesensing electrode 13, they become electrooxidized to produce sulfateions with the specific reaction depending on the type of electrolytepresent and the material making up and the relative potential of thecounterelectrode as will be more fully explained hereinafter. As theelectrooxidation of sulfur dioxide occurs, a current generated betweenthe sensing and counterelectrodes, is monitored by suitable means 15which may include amplification equipment. Although the current willdepend on the rate of diffusion of sulfur dioxide into the particularelectrolyte, temperature and pressure variations, etc., it will beevident that the current in any event will be proportional to theconcentration of atmospheric sulfur dioxide.

It is especially important that the sulfur dioxide present inelectrolyte be essentially confined to the portion of the electrolyte11a between the sensing electrode 13 and the electrolyte surface.Significant diffusion of sulfur dioxide beyond the sensing electrode 13and throughout the bulk of electrolyte 11b will be prevented duringtransducer operation since essentially all of the sulfur dioxidemolecules contacting the sensing electrode 13 will become immediatelyelectrooxidized. Accordingly, as the sulfur dioxide concentration withinthe portion of electrolyte contacting the sensing electrode 13 iscontinually diminished, more sulfur dioxide diffuses to that portion ofelectrolyte and thereafter is electrooxidized. Thus, further diffusionof sulfur dioxide into the portion of electrolyte 11b between thecounter and sensing electrodes is essentially prevented. It will beevident that sulfur dioxide present at the counterelectrode 12 wouldbecome directly reduced with no resulting current flowing between thecounter and sensing electrodes from the reaction.

As previously noted, the vessel may be open to the atmosphere or beconstructed as shown in FIG. 1 whereby a cover 18 is present. Where sucha cover is utilized, means for directing a gas into the sample gas space16 such as a gas inlet 24 and outlet 25 are provided. The cover 18 mayalso be provided with a groove or slot 26 for seating a gasket or O-ring27 which will complete the enclosure of the gas sample space 16 andconfine the gas. The cover 18 and vessel 10 may additionally be providedwith appropriate boreholes 28 through which bolts may be placed forsecuring the cover 18 to the vessel 10. Obviously, other means such asclamping devices and the like may also be used for this purpose. Thistype of construction is especially suited for directing gas streams suchas furnace or industrial stack exhausts and the like to be analyzed.

It will be appreciated that the construction of the transducer disclosedherein maybe used for analysis of a number of different gases and is notlimited to sulfur dioxide. Thus, for example, the device may be utilizedto monitor, for example, nitrogen oxides, as disclosed in our copendingapplication Ser. No. 758,35l, filed Sept. 9, 1968, concurrently herewithby proper selection of counterelectrodes and electrolyte compositions.

The sensing electrode may consist of any noble metal which itself doesnot undergo electrochemical reaction within the electrolyte. Examples ofsuitable metals include gold, platinum, palladium, iridium and the like.The electrode itself may consist of a screen, foil, porous plaque orfabricated in such other suitable form as desired. In forming anelectrode of such precious metals, as a practical manner it is oftenpreferred to form a coating of the inert metal on relatively lessexpensive metallic substrate materials. Thus, for example, a sensingelectrode consisting of a gold-plated copper or nickel expanded metal isfound to be quite satisfactory. Further, it is preferred to fabricatethis sensing electrode in a manner to expose a rather large electrodesurface area to the electrolyte solution. Accordingly, fine screens orporous electrodes may be preferred.

The counterelectrode consists of an electroactive material which iscapable of being electroreduced when in contact with the electrolyte.Where the transducer is to act as an electrooxidant-type sensor, i.e.,where sulfur dioxide is to be electrooxidized at the sensing electrode,the counterelectrode must comprise a material which will accept thenegatively charged sulfate ions beinggenerated at the sensing electrodeby the oxidizing sulfur dioxide. in turn, the counterelectrode materialutilizes the electrons and itself is electroreduced attracting thesulfate ions generated by oxidation at the sensing electrode andneutralizing them by forming a sulfate salt. Various counterelectrodecompositions may be used which are chemically compatible with theelectrolyte and relatively insoluble therein. The specific compositionof the counterelectrode will depend on how it is to be electroreducedwith the specific material selected determined by the type ofelectrolyte and the possible presence of interfering species such as theoxides of nitrogen in the gaseous mixture to be analyzed.

The electrooxidation of difi used sulfur dioxide in an aqueous acidelectrolyte solution at the sensing electrode is carried out at +0.17 v.(Stockholm Convention) relative to the standard hydrogen electrode, andwhich polarity is positive relative to the standard hydrogen electrode.The acid electrolyte in this electrooxidant-type sensor is preferablydilute sulfuric acid although other acids may be used. In such a systeman electroactive counterelectrode composition must be one itself havinga single standard reduction potential more positive than +0.17 v. at apositive polarity relative to the standard hydrogen electrode. Suitableexamples include silver chloride, antimony oxide, tellurium oxide,silver sulfate, mercurous sulfate, lead dioxide, manganese dioxide orother suitable metal oxides chemically compatible with the electrolyteand which have a reduction potential greater than +0.17 v. with positivepolarity relative to the standard hydrogen electrode. Where interferingspecies such as nitric oxide and/or nitrogen dioxide are present in thegaseous samples and which oxides will also be diffused in the aqueouselectrolyte solution, the counterelectrode composition is somewhat morelimited. This is true since the oxides of nitrogen electrooxidizes inacid electrolyte at reduction potentials of about +0.80 v. and greater,Thus, when analyzing for sulfur dioxide in atmospheres containing theoxides of nitrogen, the interference may be avoided by selectingcounterelectrode compositions having single standard reductionpotentials between +0.17 and +0.80 v. in acidic aqueous electrolytes.Mercurous sulfate, silver chloride, silver sulfate, silver chromate,silver carbonate, tellurium oxide and antimony oxide will be suitablefor this purpose.

lt will be evident to those skilled in the art that a number ofcounterelectrode compositions which will be compatible with aqueousacidic electrolyte solutions may be selected. The counterelectrodecompositions disclosed herein are in no way to be considered exhaustiveof those materials which may be used and are given only by way ofexample. However, selection of specific counterelectrode compositionsmust be made so that the electrooxidation of sulfur dioxide in theparticular electrolyte may be accomplished with the further limitationof avoiding reactions involving interferring species such as oxides ofnitrogen as set forth herein with the resulting sulfate ions beingconsumed in the reduction process at the counterelectrode by combiningwith its electroactive material to produce a chemically inactive sulfatesalt or the like.

Again, as in the case of the sensing electrodes, a number of differenttechniques for fabricating the counterelectrode may be used. Thus, thecompositions may be plated or otherwise impregnated onto inert materialsin such a manner as to allow maximum exposure of the electroactivematerial to the electrolyte solution. Where the apparatus is to be usedfor relatively long periods of time it may be desirable to cover thecounterelectrode with a suitable ion-exchange or ion-selective membranewhich will prevent the gradual buildup of dissolved metal ionsoriginating from the counterelectrode. The use of such a cover preventspossible changes of surface charac teristics resulting from extensivebuildup of ionic materials.

The gaseous mixture which is to be analyzed for sulfur dioxide accordingto the present invention may be exposed to the electrolyte solution inwhich it will be diffused by any suitable manner. Where an open vesselis used, gaseous diffusion at the electrolyte surface will readily takeplace by mere exposure to the sulfur dioxide containing atmosphere.Where an enclosed vessel is preferred, a space between the vessel walland the electrolyte surface into which space gases may be directed isnecessary. The gaseous mixture may then be pumped or otherwise fed intothe space continuously or intermittently as desired. As previouslynoted, in order to prevent extensive electrolyte evaporation asemipermeable membrane of an inert material which will not prevent orsubstantially impede gaseous diffusion of the sulfur dioxide into theelectrolyte may be used to cover the surface of the electrolytesolution. For example, Teflon polyethylene, polypropylene and the likeare suitable where the particular material may be chosen for itsrelative impermeability to possible interfering gases. The membrane willalso prevent loss of the electrolyte by spillage and will provideimproved convenience since the device may be placed in any positionduring use or storage without significant loss of electrolyte.

Monitoring of the current passing between the sensor electrode and thecounterelectrode may be accomplished by any suitable means. Although thecurrent is directly proportional to the partial pressure of sulfurdioxide present in the atmosphere diffusing into the electrolytesolution due to the relatively low-current intensity appropriateelectronic amplification will be useful. Further, equipment calibratedto read directly in parts-per-million sulfur dioxide is effective incontinually monitoring the output voltage of the amplifier althoughother suitable means may be selected.

The following examples are given to illustrate the manner in which theinvention is carried out. it is to be understood that the examples aregiven by way of illustration only and are not intended to limit theinvention to any particular or specific materials or conditions setforth therein.

EXAMPLE I A counterelectrode was prepared by mixing 6.8 g. of reagentgrade lead dioxide, 0.8 g. polypropylene powder and 0.4 g. carbon black.Approximately 4 g. of this mixture was spread evenly in a mold 1% inchby 1% inch by one-fourth inch deep having removable bottom and topplates. A flat piece of platinum mesh 1% inch by 1% inch by 0.004 inchthick was placed in intimate contact with the powder and the assemblypressurized at 6000 p.s.i.g. for 5 minutes by a hydraulic press at atemperature of 150 C. Thereafter the mold was removed from the press andcooled. The sensing electrode consisted of 3-inch diameter circularplate of gold. The electrodes were commonly wired to electronic currentamplification equipment and were assembled in a plexiglass container towhich an aqueous solution of 1N sulfuric acid was carefully added toavoid trapped air bubbles. The gold electrode was covered with aone-fourth mil thick Teflon membrane and the entire assembly madesecure. Into the air space above the membrane was continually passed anatmospheric gaseous mixture containing sulfur dioxide through an inlettube extending from the exterior of the plexiglass container. The gasexited from the enclosure through a similar projection tube opposite theinlet tube. Diffused sulfur dioxide oxidation immediately took place atthe sensor electrode with concomitant reduction of the lead dioxidecounterelectrode composition. The current caused by the two simultaneousreactions was continuously monitored while the sulfur dioxideconcentrations of the gaseous mixture entering the analyzer was changed.initially, the response time of the apparatus to indicate 90 percent ofthe actual initial sulfur dioxide concentration of 100 p.p.m. was aboutseconds. Thereafter the sulfur dioxide concentration was changed to 50p.p.m. with the recovery time of the analyzer in registering the changein concentration being about seconds.

EXAMPLE ll Mercurous sulfate was prepared by electrolysis of 6M sulfuricacid solution using a mercury pool anode and a platinum cathode with acurrent density of about 50 ma./sq. cm. The electroformed mercurysulfate was kept in suspension by gentle stirring. The product wasfiltered and washed several times with 1M sulfuric acid and dried undera vacuum. About 14.2 g. of the dried product was mixed with 0.8 g.polypropylene powder and 0.4 g. carbon black. About 10 g. of the mixturewas used in preparing the counterelectrode which was pressmoldedaccording to the procedure described in example 1.

The current collector was prepared from a silver-expanded metal. A goldsensing electrode, the counterelectrode, normal sulfuric acid, andTeflon membrane were assembled into a plexiglass container as used inexample I. A sulfur dioxide containing gaseous atmosphere in which wasalso present nitrogen dioxide was introduced into the analyzer andmonitored with results and response times being practically identicalwith that of the previous example. The presence of nitrogen oxide didnot interfere with the sulfur dioxide analysis.

We claim:

1. A transducer for measuring the concentration of sulfur dioxide in agaseous sample comprising:

a confined volume of aqueous acid electrolyte having an active surfacefor receiving sulfur dioxide molecules from said gas sample to bedissolved in said electrolyte;

a sensing electrode immersed in said electrolyte adjacent the activesurface consisting of a chemically inert material for contacting thesulfur dioxide dissolved in said electrolyte for electrooxidation toproduce sulfate ions;

means, including a counterelectrode of a material selected from thegroup consisting of lead dioxide, mercurous sulfate and silver sulfateimmersed in said electrolyte and spaced from said sensing electrode onthe side opposite said active surface of the electrolyte, electricallycoupled to said sensing electrode for maintaining said sensing electrodeat a predetermined positive potential of more than 0.17 v. relative tothe standard hydrogen electrode and less than approximately 0.80 v.relative to the standard hydrogen electrode at which other gases in thesample may be oxidized by said sensing electrode and for maintainingsaid counterelectrode at a positive potential relative to said sensingelectrode; and,

indicator means electrically coupled for measuring the amplitude of thecurrent flow between said sensing electrode and said counterelectrode.

2. The transducer of claim 1 wherein: the surface of said sensingelectrode consists of a material selected from the group consisting ofgold, platinum, palladium and iridium.

3. The transducer of claim 1 wherein: said counterelectrode consists oflead dioxide.

4. The transducer of claim 1 wherein: said counterelectrode consists ofmercurous sulfate.

5. The transducer of claim 1 wherein: said electrode consists of silversulfate.

6. The transducer of claim 1 further comprising:

membrane means for covering said active surface and confining saidelectrolyte consisting of a membrane material readily permeable to thediffusion of sulfur dioxide therethrough and relatively impermeable tosaid electrolyte.

7. The transducer of claim 6 wherein: said membrane material is selectedfrom the group consisting of Teflon and polyethylene.

8. The transducer of claim 6 wherein: said membrane material is Teflon.

2. The transducer of claim 1 wherein: the surface of said sensingelectrode consists of a material selected from the group consisting ofgold, platinum, palladium and iridium.
 3. The transducer of claim 1wherein: said counterelectrode consists of lead dioxIde.
 4. Thetransducer of claim 1 wherein: said counterelectrode consists ofmercurous sulfate.
 5. The transducer of claim 1 wherein: said electrodeconsists of silver sulfate.
 6. The transducer of claim 1 furthercomprising: membrane means for covering said active surface andconfining said electrolyte consisting of a membrane material readilypermeable to the diffusion of sulfur dioxide therethrough and relativelyimpermeable to said electrolyte.
 7. The transducer of claim 6 wherein:said membrane material is selected from the group consisting of Teflonand polyethylene.
 8. The transducer of claim 6 wherein: said membranematerial is Teflon.